A variety of technologies have been developed for transmitting power and or signals (such as data signals from sensors) to/from deep underground in hydrocarbon production wells. The use of advanced extraction technologies, such as Multi-lateral wells and Through Tubing Rotary Drilling (TTRD) increases access to hydrocarbon reserves through existing well completions by drilling new sidetracks branching off the existing production tubing. The distances involved in these well branches can extend to many kilometers. This poses significant problems for installing signal and power transmission systems. Most current systems are based on wired power supply from surface. However, the cables in cable systems are especially vulnerable to damage, and there are difficulties in making the cable connections inside the well.
Other techniques include inductive coupling with the use of coupled loop antennas. Another such technology, which is described in WO2007/004891, involves the use of “current transformers” or inductive couplers to induce a current onto the production tubing and pick it up again from the tubing. However, existing inductive coupling technologies have a major problem if there is a short circuit between the inside of the tubing and the annulus fluid along a long length of transmission. Another problem with this type of system is that sending a signal up the tubing to a receiver at the wellhead often requires the use of many repeaters. Repeaters add to the complexity and installation cost, as well as limiting access to the well bore. Indeed, a major drawback with a current transformer is that it requires a substantial mass of inductive material as well as a large number of windings on the secondary coil and/or a relatively high frequency to effectively pass a reasonable amount of power.
There is also a trend in new oil fields to have many well branches that finger out into the reservoir from one or several mother wells. These well patterns can benefit from the use of electromagnetic techniques to determine properties including hydrocarbon saturation in-between the branches, so an effective, or improved method to radiate and receive electromagnetic waves is attractive.